Styrenic (meth)acrylic oligomers prepared by continuous bulk polymerization of vinylic monomers at high temperatures are low molecular weight copolymers which contain some residual, terminal vinylic unsaturation, or carbon-carbon double bonds. Such residual unsaturation may adversely impact the stability and other properties of these oligomers and products and articles made from them. Residual unsaturation may reduce the thermal stability of the styrenic (meth)acrylic oligomers and limit their utility in certain applications that require exposure of these polymers to high temperature conditions.
For example, high glycidyl methacrylate (GMA)-content, styrene-GMA oligomers, which are made under high temperature bulk polymerization conditions, are excellent chain extenders for a number of plastics, such as, polyethylene terephthalate (PET), polylactic acid (PLA), polycarbonate (PC), and PET copolymerized with cyclohexane dimethanol (PETG). However, due to their reduced thermal stability, the styrene-GMA oligomers are not suitable for certain applications that require contact with food.
For applications where styrenic (meth)acrylic oligomers are to be in contact with food, strict guidelines must be met in terms of the presence of residual monomers in the final article. Because of such strict restrictions on residual monomers, polymeric additives should fulfill two basic requirements: they should have very little or no residual monomers to begin with, and have minimal or no generation of monomers and other harmful chemicals during the compounding and making of a final article.
In typical styrene-GMA oligomer applications, small amounts of the oligomers are compounded with the host plastic to make the final articles, such as bottles. Compounding temperatures range from 200° C. to 220° C. for PLA, and can go as high as 270° C. or even higher for PET. The compounding cycle usually lasts for 5 minutes, or less. However, under such conditions, styrene-GMA oligomers may begin degrading. At the high temperatures used for making styrenic (meth)acrylic oligomers, terminal double bonds or terminal vinylic unsaturation are produced. These terminal unsaturations are one of the reasons that such styrenic (meth)acrylic oligomers in general, and the high-GMA containing styrene-GMA oligomers in particular, may be thermally unstable. Conventional styrene-GMA oligomers may start degrading at temperatures in the 200° C. to 250° C. range. Because of the thermal instability of the styrene-GMA oligomers, and their concomitant degradation, their use in products that have direct contact with food products is limited, as well as their use in numerous other applications.
Conventional, high-temperature produced (meth)acrylic oligomers have similar draw-backs, but not necessarily for the same reasons. For example, where the (meth)acrylic oligomer is an acrylic-based oligomer, some amounts of unsaturation may be present, leading to thermal instability as above with respect to the styrene-GMA oligomers. For methacrylate oligomers, little unsaturation is present, however, high-temperatures impart some inherent instability. All methacrylate systems are not polymerized at high temperatures due to the thermodynamics of polymerization but the resulting polymers still have disadvantages when used at high temperatures or under demanding conditions.
The presence of residual unsaturation in conventional styrenic (meth)acrylic and acrylic oligomers may lead to ultraviolet (UV)-light absorption by the oligomers and subsequent degradation of the oligomers or products such as coatings containing these oligomers.